This invention relates generally to human hip replacement, and more particularly to achieving more accurate leg length restoration and joint stability, as during surgery. The invention concerns implantable orthopedic prostheses for total hip replacement and, more particularly, a prosthesis which receives a modular neck assembly selected for a desired anteversion, neck length, neck angle and offset.
During such surgery, a replacement stem is employed and inserted lengthwise into a pathway in the femur. The stem carries an angularly extending i.e. offset neck, and a ball at the neck terminus, to be received into a socket defined by the joint. These components must be accurately relatively positioned to accommodate to each patient's particular femur and hip socket configuration, as during surgery, which is time consuming and subject to adjustment problems and difficulties. It is found that the stem may not seat at a local predicted by the surgery, and that dislocation may occur resulting from use of decreased offset (lateral neck spacing between ball and stem), and/or from positioning the stem such that the neck does not well align with the socket. Avoiding such dislocation may result in undesired compromises between desired leg length (determined by degree of stem insertion into the femur recess) and overall joint stability.
More specifically, in hip replacement surgery, the natural head and neck portion of the femur are removed and replaced with a metallic hip prosthesis called a stem. The stem is inserted in a cavity formed in the femur. This prosthesis generally comprises three elements: a distal stem portion for fixation into the distal part of the femur, a proximal body portion for fixation in the metaphysis of the femur, and a neck portion for replacing the natural femoral neck which is formed at an angle to the stem. These elements can be connected and configured in numerous ways, but generally these elements form either a one-piece stem design or a modular stem design. The neck typically has a male taper at its free proximal end and is mated with a spherical ball (head) that has a female taper machined into it. The surgeon can choose heads of various diameters with either deeper or shallower tapers to adjust neck length. The head is mated with a socket in the hip. Every patient requires some degree of fitting due to the unique anatomical requirements of the particular patient. Also, often, the stem is either inserted too far or too short into the femur, many times because of the mismatch between the broach that prepares the femur to receive the stem and the stem. If the stem is inserted too far into the femur, then the leg length is shortened. On the other hand, if the stem is not inserted far enough into the femur, then the leg length is lengthened. The surgeon then has to make adjustments with the head to correct this. This adjustment occurs independent of the position of the stem in order to restore and obtain the proper leg length.
One-piece designs are typically formed from a solid piece of metal, such as titanium, stainless steel, or cobalt chromium alloys. As such, the stem, proximal body, and neck are integrally formed together. Even though the implants are manufactured in a wide range of shapes and sizes, the individual elements cannot be separately altered or sized since no changes or adjustments occur between the elements themselves.
The largest cause of early failure and revision surgery in hip replacement is dislocation of the head from the socket caused by instability of soft tissues surrounding the hip joint which is caused mostly by lack of head center distance to that of the femoral axis (called offset) or improper alignment of the femoral stem relative to the acetabular cup. This prevents the surgeon from being able to stabilize the joint adequately. The joint could be tensed by either using stems that offer adequate offset or the surgeon can use heads that would lengthen the leg which is not preferred. In an effort to resolve this, manufacturers are offering stem systems with smaller and larger offsets. Some achieve the offset by changing the neck angle. This method is not well accepted as the leg length cannot be maintained especially when heads with shallower taper are used to adjust leg length. The favored method of increasing offset is using systems that provide parallel shift of necks, i.e. the neck angle is preserved while the location of the neck is shifted further away from the centerline of the femoral axis. In an effort to reduce inventory and provide surgeon with more flexibility manufacturers are offering various modular neck and stem components. Another issue with unitized designs is inability to match the neck on the stem with the patient's natural femoral neck anteversion (forward rotation) that is widely variable from patient to patient. An inaccurate anteversion can cause a decrease in range of motion, neck impingement, excessive component wear, and lead to subluxation or even dislocation.
In contrast to one-piece designs, modular designs have some components that are interchangeable. Specifically, modular hip prostheses are formed from individual, separate components that are interchangeable and connectable together. The amount of modularity and degree of adjustability between components varies widely depending on the design and manufacturer of the prosthesis. A prosthesis in which the stem and neck are a unitary device requires that the surgeon have a large quantity of prostheses available to provide correct bio-mechanical function of the prosthesis with the patient. It is very costly to manufacture and maintain a large inventory of prostheses and, despite the number of prostheses available, quite often the appropriate stem size does not provide the neck offset, angle or anteversion required to best fit the patient.
One of the first modular designs in production is shown in U.S. Pat. No. 4,846,839 entitled “APPARATUS FOR AFFIXING A PROSTHESIS TO BONE” to Noiles. In one embodiment, a proximal body that connects to a distally fixed to bone stem and neck integrally formed together is described. The stem and neck component is angularly adjustable to the proximal sleeve allowing for 360 degree version control. Also different stem and neck components are possible which allow for horizontal offset shift which is favored by surgeons. However use of such system would require a perpendicular femoral neck resection for the placement of the proximal sleeve and for the surgeon to be able to clear femoral bone when placing the stem and neck portion in a rotated position. A perpendicular neck resection would result in a larger bone removal from the neck, which has been shown to reduce proximal fixation and initial stability of the stem in bone which is not favored. As such, this type of design has to augment proximal fixation with distal fixation and load transfer.
More recently, modular prostheses have been designed to overcome the low neck resection problem. Ceramascoli describes in U.S. Pat. No. 4,957,510, “HIP PROSTHESIS STRUCTURE ADAPTED FOR EASY FITTING TO PATIENT COXO-FEMURAL ARTICULATION”, a modular neck and stem configuration. In such design the neck resection angle is oblique and preserves femoral bone which provides a favorable situation by preserving bone and achieving more of a proximal fixation. However, the tapered junction does not allow design of a neck portion that can provide parallel offset shifts. Many manufacturers have introduced designs with this feature and are forced to accept offset increase by method of neck angle change which is not favored by surgeons. As such these designs have not been well accepted in the market.